drm/tegra: hdmi: Silence deferred-probe error

Driver fails to probe with -EPROBE_DEFER, which produces a bit noisy error
message in KMSG during kernel's boot up. This happens because voltage
regulators tend to be probed later than the DRM driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>